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INTRODUCTORY   ESSAY.                                     77
The whole of Continental India lies north of the equator,
and considerably more than half of its area north of the
Tropic of Cancer, whose position very nearly corresponds
\vith the hase of the peninsula of Hindostan. Proceeding
northwards from the tropic, there is no sea nearer than the
Arctic Ocean; but as we advance towards the equator the
width of the land gradually diminishes both in the Madras
and Malayan peninsulas. It may be observed also, that due
south of India, the ocean extends without interruption be-
yond the Antarctic Circle, while to the eastward, not only
on the equator but in the southern hemisphere, there is
much land. The  Eastern Archipelago, from consisting of
Hrge islands, separated by belts of s'ca, possesses a humid and
equable climate; but the great continent of Australia, being a
vast expanse of low land, becomes enormously heated when
the sun is in the southern hemisphere, and presents extremes
of climate. To the westward the coast-line of Beluchistan
continues somewhat north of the tropic till it enters the Per-
sian Gulf; but the great continent of Arabia advances far
within the tropic; while, a little further west, Africa extends,
uninterrupted by sea, far into the south temperate zone. From
ibis relative position of land and sea, it is evident that the
whole of the rain which falls in India must be derived from
the southward or eastward, and that those parts only can be
subject to heavy rains, towards winch the sea-wind blows.
The maps of the monthly isotheruials*, recently published
by Dove, enable us to trace with considerable accuracy the
periodical changes of temperature throughout India and the
neighbouring countries. An inspection of these maps shows
us that iii January the isothermal imes in the northern he-
misphere arc nearly parallel to the equator, but that, in the
southern, Africa and Australia arc pvctcrnaturalljo hot. Till
the vernal equinox, the equator of heat (or that line from
which the temperature diminishes both towards the north awl
towards the south) lies south of the terrestrial equator; but
Tsothewuals appondul to this Kssio.